Cable television is typically provided by broadcasting television programming over a community access television (CATV) network including a hybrid fiber coax (HFC) cable system. In the United States, a forward passband, 50 MHz to 550 MHz, is allocated for transmission of CATV signals containing program material from a headend of the HFC cable system to the premises of subscribers or end users. Such transmission of the CATV signals from the headend to the end user premises is said to be in a “forward” or “downstream” direction.
In the “reverse” or “upstream” direction, an end user may use a terminal to transmit data to the headend to realize interactive services such as interactive games, telephone services, home shopping and Internet access. In the United States, the upstream data is transmitted through a reverse passband, 5 MHz to 42 MHz, of the HFC cable system. The terminal used by the end user is connected to, or incorporates, a cable modem for modulating a specified carrier with the data to generate a radio frequency (RF) signal representing the data. The RF signal populates a channel corresponding to the carrier in the reverse passband.
FIG. 1 illustrates a prior art arrangement including an HFC cable system with only those components relevant to upstream data transmission being shown. In FIG. 1, an end user may use terminal 100, e.g., a set top terminal or computer with a cable modem, to transmit data upstream to headend 140. In a well known manner, terminal 100 generates an RF signal representing the data, which populates a channel in the reverse passband. The RF signal from terminal 100 is combined by combiner 110 at a distribution tap with other RF signals, which contain data from other terminals and populate other channels in the reverse passband. The combined analog upstream signal is transported through cable distribution network 115 to distribution node 120. Linear optical transmitter 125 in node 120 receives the combined signal from link 117 extending from network 115. Transmitter 125, incorporating a linear laser, converts the combined signal into an optical signal. The latter traverses optical fiber 130 to headend 140. Linear optical receiver 135 in headend 140 converts the optical signal back to the combined signal. Cable modem termination system (CMTS) 150 processes the combined signal to recover the data from the respective terminals, and reformats the data in proper formats for further transmission to appropriate servers to realize various interactive services.
In the cable industry, the Data-Over-Cable Service Interface Specifications (DOCSIS) are well known, which specify, among others, the data format of the upstream transmission, and the methods of, and equipment used in, the transmission. According to the DOCSIS, each user terminal can be programmed to transmit on a carrier frequency and through the corresponding channel specified by the CMTS. The carrier frequencies are selected to optimize the upstream communication by avoiding different types of noise, power losses or other problems occurring at known frequencies in the reverse passband. As a result, the reverse passband can accommodate up to 7 or 8 DOCSIS compatible channels.
As additional interactive services are being introduced and more and more users are subscribing to the interactive services, it is anticipated that the HFC cable system capacity for upstream communication, which is limited, will soon be overly strained. Accordingly, it is desirable to improve the traditional upstream communication technique to effectively utilize the limited HFC cable system capacity.